In recent years, environmental problems caused by waste plastics have come to the fore, and the realization of a recycling-based society on a global scale has been desired. Under the circumstances, biodegradable plastics have received attention which are decomposed into water and carbon dioxide after use by the action of microorganisms. Further, in Europe etc. where kitchen garbage is treated by composting, there has been a demand for garbage bags that can be put in compost together with garbage. Examples of biodegradable plastics include poly(3-hydroxyalkanoate) (hereinafter, referred to as P3HA), polycaprolactone, polybutylene adipate terephthalate, polybutylene succinate adipate, or polybutylene succinate, and films and sheets using these biodegradable plastics have been developed. Particularly, among these biodegradable plastics, P3HA has the most excellent biodegradability, and therefore can be treated by various biodegradation processes such as composting at about ordinary temperature and anaerobic degradation. Therefore, from the viewpoint of biodegradability, it is preferred that P3HA is used singly or a resin composition containing a high proportion of P3HA and another biodegradable resin is used.
As conventional film and sheet excellent in biodegradability, for example, a film comprising poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (abbreviation: PHBH) produced by a microorganism (see Patent Document 1), and a sheet comprising a resin composition containing a petroleum-based resin, such as polybutylene adipate terephthalate, polybutylene succinate adipate, polybutylene succinate, or polycaprolactone, and PHBH (see Patent Document 2) have been disclosed.
However, when an aliphatic polyester produced by a microorganism, such as PHBH, or a resin composition containing such an aliphatic polyester is formed into a film or sheet by film blowing or T-die extrusion, the resulting film or sheet is poor in MD-tear strength.
As means for improving the rigidity and toughness of a biodegradable and/or biomass-derived resin, blending the resin with surface-coated inorganic particles to obtain a composition has been disclosed (see Patent Document 3).
However, polylactic acid disclosed in Patent Document 3 is originally poor in tear strength, and therefore its tear strength is still poor even after blending with inorganic particles. On the other hand, unlike polylactic acid, P3HA tends to have a lower molecular weight in an acid or basic environment, and therefore it is difficult to improve its tear strength by using coated inorganic particles.
Blending with silica has also been disclosed. For example, blending of an aliphatic polyester resin and an aromatic-aliphatic polyester resin with inorganic particles having a specific particle size to obtain a composition having improved tear strength has been disclosed (see Patent Document 4). However, the tear strength of the composition is insufficient to continuously produce a film or sheet. Further, there is no description about a case where the aliphatic polyester resin is P3HA and a case where the inorganic particles are silica particles. Further, a composition obtained by blending a polylactic acid-based aliphatic polyester resin, a plasticizer, and hydrophilic silica has been disclosed (see Patent Document 5). However, blending with the hydrophilic silica is intended to improve heat resistance to the polylactic acid-based resin, and there is no disclosure about improvement in the tear strength of a resin comprising P3HA and PBAT. Also, a composition comprising silicon dioxide, a plasticizer, and a biodegradable resin, such as polylactic acid, has been disclosed (see Patent Document 6). However, blending with the silica is the technique of uniformly dispersing the plasticizer in the rigid biodegradable plastic to improve flexibility, and therefore the silica is used as a carrier of the plasticizer to uniformly disperse the plasticizer. On the other hand, the present invention is applied to soft biodegradable plastics, and therefore does not have such a problem and is different in technical idea. Further, there is no disclosure about improvement in the tear strength of a resin comprising P3HA and PBAT which is the object of the present invention.
Further, a composition comprising polyhydroxybutyrate that is one of P3HAs, a plasticizer, and a crystal nucleating agent has been disclosed (see Patent Document 7). However, blending of the resin having a high melting point with the plasticizer is intended to suppress thermal degradation during processing, and therefore tear strength is poorly improved.